1. Field of the Invention
The present invention relates to a magnetic toner having at least magnetic resin particles and a surface-modified fine silica powder, contained in a developer for developing an electrostatic image to convert the electrostatic latent image to a visible image in image forming methods utilizing electrophotography, static recording, static printing or the like. It also relates to an image forming method making use of such a magnetic toner. More particularly, the present invention relates to a magnetic toner suited for high-speed image formation using an amorphous silicon drum as an electrostatic image bearing member.
The present invention is also concerned with a surface-modified fine silica powder preferably used as an additive for a developer for developing an electrostatic image to convert the electrostatic latent image to a visible image in image forming methods utilizing electrophotography, static recording, static printing or the like. More particularly, the present invention is concerned with a surface-modified fine silica powder suitable as an additive for a developer used in high-speed image formation using an amorphous silicone drum as an electrostatic image bearing member.
2. Related Background Art
A method commonly known as an image forming method that carries out electrophotography is a method in which, using a photosensitive drum as an electrostatic image bearing member, the surface of the photosensitive drum is uniformly charged by a charging means such as a corona charging assembly. The drum is then imagewise exposed to light to form an electrostatic latent image on the surface of the photosensitive drum, and the electrostatic latent image is developed by a developing process such as jumping development or magnet brushing, using a developer having a magnetic toner, to thereby form a toner image on the photosensitive drum surface. The toner image is further transferred to a recording medium and then fixed thereon.
Developers used for converting the electrostatic latent image on the surface of a photosensitive member to a visible image include two-component developers comprised of a mixture of a magnetic carrier such as iron powder or ferrite powder and a toner having a resin and a colorant and one-component developers that make use of no carrier.
In the development making use of the two-component developers, the quality of toner images greatly depends on the mixing ratio of toner and carrier, i.e., the toner concentration in a two-component developer, and hence the toner concentration in the developer must be controlled so as to be always constant, making the process troublesome.
On the other hand, compared with the above two-component type developing system, the development using the one-component developers, which makes use of magnetic toners mainly composed of a resin and magnetic powder, has the advantages that no device for keeping the toner concentration constant is required which makes the process easy to use and also developing apparatus can be made small-sized.
In the system in which a one-component developer having a magnetic toner is used, the developer that must be coated on a developer carrying member, a sleeve, in a thin layer, tends to be coated on the sleeve in a very large thickness to cause background fogging. The phenomenon of agglomeration of magnetic toner gradually begins to occur with an increase in charges of the magnetic toner. In particular, such a problem tends to occur in high-speed copying machines which take copies on 50 sheets or more per minute.
To solve such a problem, Japanese Patent Application Laid-open No. 55-120041 discloses a method in which an insulating magnetic toner is made to contain fine silicon dioxide particles with a pH of 7 or more, having a trimethylsiloxyl group, i.e., hydrophobic fine silica powder. Addition of such hydrophobic fine silica powder brings about a tendency toward control of an increase in charges of the insulating magnetic toner. However, in the high-speed copying machines, such addition brings about an increase in charges in a low-humidity environment to sometimes cause problems such as a decrease in image density and the background fogging.
The fine silica powder when made hydrophobic tends to form agglomerates when so treated, and may often form agglomerates of several hundred .mu.m in size during the treatment. Such agglomerates inhibit chargeability of toners to cause a decrease in charges of toners. Moreover, agglomerates with such a large particle size have so small a specific surface area (m.sup.2 /g) and so weakly interact with toner particles that they tend to be separated from the toner particles. Hence the agglomerates tend to scatter alone from a developing assembly.
The agglomerates having scattered therefrom have so small a specific gravity that they fly about inside a copying machine according to air currents inside the copying machine and reach a discharge wire used for corona charging, so that the wire of the charging assembly is soiled. The part at which the wire has been soiled gives a weak corona discharge to make non-uniform the charge distribution of the corona charging assembly, so that images formed tend to have a density uneveness. The soiling of wire with silica is not a problem peculiar to one-component developers, and is a problem that may be also caused in two-component developers. Japanese Patent Application Laid-open No. 60-107036 discloses, as a method by which the discharge wire is better prevented from being soiled with fine silica powder, a method in which the fine silica powder is controlled to have a bulk density of not more than 30 g/lit and is added in a developer in a smaller quantity so that any difficulty that may occur when added in a large quantity can be lessened or relieved. However, although this method can be effective for relieving the difficulty, the problem of the soiling of wire caused by the addition of fine silica powder still substantially remains, and hence the discharge wire is soiled as a result of repeated copying on several tens of thousands of copy sheets.
In particular, an amorphous silicon photosensitive member having a superior durability required for photosensitive members is excellent as a photosensitive drum used for high-speed copying machines. In order to maintain the dark portion surface potential, however, it requires a corona discharge current having a larger volume, exceeding 500 .mu.A, than photosensitive members of other types. Thus, the discharge wire used therefor tends to be soiled more with fine silica powder.
As stated above, it is sought to provide a durable fine silica powder of excellent durability capable of being used in developers for high-speed copying machines.